The present invention relates generally to composite articles made from composite materials and which comprise protective sheets, as well as methods of making and using the same.
A variety of protective sheets and protective coatings can be selectively applied to protect exterior surfaces on a variety of articles. Often, protective sheets and protective coverings are selectively applied to an article's surface after the article is formed. Many such applications are intended to reduce abrasion or other wear on the underlying surface. Other applications are geared primarily toward maintaining or improving aesthetics or integrity of an underlying surface (e.g., when the underlying surface contains printed material thereon or when the surface contains aesthetically undesirable pinholes, bubbles, or other surface imperfections that can occur when, for example, producing molded articles). It is not surprising that there exists a need for protective sheets and protective coatings in a variety of applications.
Certain protective sheets and protective coatings, as well as related methods for making and using the same, are known. As an example, polyurethane “clearcoat” protective coatings have been applied to a variety of finished molded parts, such as plastic body panels, in order to bolster weathering performance, as well as scratch- and abrasion-resistance, without detrimentally affecting appearance of the underlying surface. Another example involves application of polyurethane sheets to automotive body panels for protection of the body panel against chipping or other damage caused by, for example, objects such as flying stones or debris. As another example, “leading edge tapes” have been applied to select portions of articles, such as helicopter blades and aircraft noses or wings, in order to protect underlying surfaces from scratch- and abrasion-resistance.
When the underlying surface is made from a composite material (e.g., a fiber-reinforced composite), surface protection often has multiple goals within a particular application. A wide variety of composite materials and methods for their preparation and use are known. In the case of a fiber-reinforced composite, a polymeric resin matrix and fibrous reinforcement together often form the composite. A variety of materials can be used for each of the polymeric resin matrix and the fibrous reinforcement components. For example, materials useful for fibrous reinforcement include carbon fibers, boron fibers, and glass fibers. Further, examples of materials useful for the polymeric resin matrix include thermoplastics (e.g., nylon) and thermosets (e.g., epoxies and phenolics).
Composite materials are finding increased use in applications where lightweight materials are desired and where an associated compromise in strength or stiffness of the material would likely be problematic. Many composite materials are also useful in applications where corrosion resistance is desired, as composite materials more often exhibit excellent corrosion resistance as compared to alternative materials.
Due to their beneficial properties, a variety of specialized sporting implements and other articles are increasingly being made from composite materials. For example, composite materials are increasingly being used in shaft-based sporting implements (i.e., those sporting implements having a generally elongated portion, which may or may not be hollow or uniform in thickness and shape throughout) and similar articles. Such articles include, for example, golf clubs, bicycle frames, hockey sticks, lacrosse sticks, skis, ski poles, fishing rods, tennis rackets, arrows, polo mallets, and bats. As an example, the use of composite materials enables golf club manufacturers to produce shafts having varying degrees of strength, flexibility, and torsional stiffness.
In addition, a variety of articles in the transportation and energy industries are increasingly being made from composite materials. For example, composite materials are often used to make various aerospace components, such as wing and blade components, including those on helicopters and specialized military aircraft. Further, composite materials are often used to make various automotive components, both interior and exterior, including body panels, roofs, doors, gear shift knobs, seat frames, steering wheels, and others. In the energy industry, composite materials are used to make wind mill blades—e.g., large wind turbine blades are made more efficient through the use of carbon fiber-reinforced composites. Indeed, the number of current and potential applications for composite materials is extensive.
Beneficially, composite materials offer enhancements in strength, stiffness, corrosion resistance, and weight savings. These beneficial properties are often balanced against competing relative weaknesses in abrasion resistance and impact resistance. In addition, as many composite articles are made by layering multiple, individual composite material layers to achieve the desired properties, such composite articles are susceptible to interlayer delamination, particularly upon impact. This is especially the case with carbon fiber-reinforced composites (also referred to as “CFR composites”). When interlayer delamination occurs, structural integrity of such articles is compromised, sometimes leaving the composite article useless as intended. Further, in extreme cases where the composite article fractures, a sharp broken surface can result (i.e., with reinforcing fibers extending haphazardly therefrom), which impacts not only the usefulness of the article, but also the safety of those using such articles and those around them. Thus, breakage prevention and containment are also important design factors.
In order to enhance certain properties of composite articles, gel coats or similar protective coatings have conventionally been used. Gel coats often impart a glossy appearance and improve other aesthetic properties of the article. In addition, gel coats can provide some, although limited, enhancements in abrasion resistance. Gel coats or similar protective coatings are conventionally applied to composite articles that are formed by molding for, if no other reason, aesthetic enhancement. Particularly when molding articles from composite materials, surface imperfections are likely to develop, giving rise to a need for aesthetic enhancement. One mechanism for the increased number of surface imperfections in molded composite articles is associated with tiny air bubbles forming at the interface with the mold when the polymer matrix of such composites does not sufficiently flow throughout the reinforcement (e.g., fibers) during molding. The result is that the surface of the composite article, which is formed against the face of the mold, contains imperfections that can detract from a glass-like appearance otherwise desired.
There are two widely used methods of applying gel coats or similar exterior protective coatings to composite articles. The first method involves spraying the gel coat onto an exterior surface of a composite article after the article is formed (e.g., by molding). The second method involves eliminating this subsequent processing (e.g., post-molding) step by pre-applying the gel coat to the interior surface of, for example, a mold where it can then be transferred to an exterior surface of the composite article formed therein. For example, see U.S. Pat. Nos. 4,081,578; 4,748,192; and 5,849,168. This method, which is one variation of “in-mold processing,” is sometimes referred to as in-mold declaration or in-mold labeling depending on the application and materials used. Another variation in the use of in-mold processing for application of materials, although complicated and inefficient, is described in U.S. Pat. No. 5,786,285.
While gel coats are capable of improving the aesthetics of surfaces to which they are applied, they are often not capable of imparting some or all of the desired performance properties. For example, gel coats are often too thin or too hard to provide substantial levels of abrasion resistance. Further, gel coats typically do not provide significant impact resistance when used on composite articles. After extended use, gel coats also have a tendency to crack, which enables water to penetrate into articles on which they are applied. Over time, such water penetration may lead to significant structural damage of the composite article. When the article is subjected to freeze-thaw cycling (e.g., as with many aerospace parts that undergo several freeze-thaw cycles in a single day of operation), premature structural failure is often more rapid, as any water trapped within the composite article will likely produce larger cracks and similar internal damage based on such freeze-thaw cycling.
In addition to their inability to often provide desired performance properties, use of gel coats typically decreases overall processing efficiency. For example, if the gel coat is spray-applied to a surface in a post-processing step, additional labor and manufacturing time is required in conjunction therewith. Even when applied in-mold, for example, typical gel coats require cure time after application to a mold surface and before actual molding of the composite article. Such cure time can take several hours, which is obviously undesirable from the perspective of processing efficiency.
While in-mold processing is otherwise generally more efficient than post-mold application of gel coats, if printed material (e.g., a textual or graphical decal) is applied to the surface of a finished composite article, a gel coat must then be conventionally spray-applied to that surface or a protective coating or protective sheet must generally be applied over the printed material as a post-processing step. This is often necessary even if a gel coat has already been applied to the surface in-mold.
The types of materials that can be applied as a gel coat are also limited, which is undesirable as it decreases flexibility in design and manufacture of composite articles. For example, many conventional gel coat materials are two-part compounds having a relatively short pot life, which requires that they be used within a few hours of compounding or discarded. When in-mold application of gel coats is desired, additional constraints must be considered. For example, availability of certain polymer matrix systems for in-mold processing, such as those based on epoxy thermoset resins used with carbon fiber reinforcements, is very limited.
It should be noted that materials other than gel coats have been applied “in-mold” and to different types of underlying surfaces. For example, multi-layer paint replacement film has been converted to a finished product through an in-mold decoration process. This process typically involves back molding of the film to form a finished article having the paint replacement film integrally adhered to the outer surface. In-mold processing has also been utilized to construct certain specialized sporting implements such as bicycle helmets, where a foam layer is in-mold bonded to the hard outer shell of the helmet. Nevertheless, application of exterior protective coverings to surfaces, particularly those exterior surfaces formed from composite materials, is in need of improvement.
To provide higher levels of abrasion resistance or impact resistance beyond what gel coats alone can provide, protective sheets have been added to the exterior surfaces of composite articles in addition to gel coats. As compared to a protective coating, such as a gel coat, a “protective sheet” is generally applied to a surface in its cured form. In contrast, a coating is generally applied to a surface to be protected in an uncured (e.g., solution) form, after which it is cured in-situ. While a sheet may be formed using conventional extrusion, casting, or coating technology, before the sheet is applied to a surface to be protected it is cured and/or formed.
Conventional protective sheets are often applied to a surface using a pressure sensitive adhesive. Many undesirable issues can arise, however, if the pressure sensitive adhesive is not adequately designed and formulated. For example, many pressure sensitive adhesives lack adequate bond strength to prevent edge lifting of protective sheets that are adhered to an underlying surface using the same. Protective sheets applied using existing technologies are typically not permanently bonded to the underlying surface. The durability of such constructions is often short-lived, as the adhesive bond often fails during repeated use, causing the protective sheet to lift from the surface. As another example, many conventional pressure sensitive adhesives are either repositionable and/or removable. This allows conventional protective sheets, which may lack adequate extensibility for easy application to a surface (especially irregular-shaped surfaces), to be more easily applied to surfaces. However, such pressure sensitive adhesives typically lack adequate permanency. In addition, often when a pressure sensitive adhesive is used for bonding a protective sheet to an underlying surface, a gel coat or other protective coating is often used in addition to the protective sheet (e.g., a coating is applied to an underlying surface before the protective sheet is applied).
In addition to the shortcomings associated with bonding of protective sheets to an underlying surface, application of protective sheets has proven to be otherwise difficult. For example, in addition to the bonding issues arising based on the often inadequately extensible nature of conventional protective sheets, it is often difficult to apply protective sheets to surfaces with complex shapes when relatively thick or multi-layer protective sheets are used. As a result, wrinkles often exist in protective sheets so applied. Even if uniformly applied to irregular surfaces initially, over time conventional protective sheets are prone to lifting from such surfaces. In any event, the way in which protective sheets are typically applied to such surfaces generally decreases processing efficiency.
While some benefits can be obtained from application of protective sheets and protective coatings according to known methods, such conventional methods often result in composite articles that still fail to adequately address important performance and processing considerations. Not only are performance property considerations important, but for the reasons stated above, aesthetics are also often another important consideration.
When attempting to address the myriad of important considerations, however, processing efficiency is often compromised. This is the case when, for example, multiple protective sheets and/or protective coatings (e.g., gel coats) are applied to a surface. In order to improve processing efficiency, it is desirable to minimize the number of protective sheets and protective coatings such as gel coats (especially those gel coats used primarily for aesthetic enhancement) that are applied to protect surfaces of underlying composite articles. For example, if gel coats could be eliminated, processing efficiency could improve both in terms of cost and time savings associated with the otherwise required additional processing steps associated with gel coating.